As multi-media displays and audio/video processing devices increasingly transition to higher performance technologies that support more functionalities and features, the problem of interfacing becomes more complex. The challenge is to establish reliable communications with a minimum number of channels that meet the performance and cost objectives of the application. Known interface technologies, such as VGA, component video, DVI, HDMI, DisplayPort, and SDI provide several examples of current multi-media interconnect technology. These known technologies, however, suffer from several limitations.
For example, VGA is limited in its ability to scale to support higher resolutions and color depths, and it does not have a means available to protect high definition content for consumer use. In addition, VGA does not provide native multi-media connectivity, being limited to video and graphics with limited control via the Display Data Channel (DDC) interface. The VGA connector and cabling is also physically very large, difficult to route, and relatively expensive.
Analog component video, like VGA, suffers from similar scalability issues and lack of true multi-media support, although there is a defined means (Macro Vision™) to protect content for consumer use. The Macro Vision™ technique, however, provides only a limited level of protection. The component connectors and cabling provide probably the best performance of all existing interfaces in terms of cost, routability, and signal integrity. In addition, like VGA, long cable runs of many 10's of meters are possible with component video before any noticeable signal loss becomes apparent.
DVI technology is limited in that the DVI 1.0 specification is in essence frozen and cannot be easily updated to support higher clock speeds, color depths, connector types, or new features. Although DVI does provide optional content protection, there is no support for audio. DVI connectors and cabling are also physically very large and difficult to route, and the cable length is extremely limited and is very expensive. Moreover, interoperability issues have plagued this interface, which has contributed to the relatively low adoption rate for DVI, even in the target application of PC to monitor interfaces. Another limitation of DVI is that the physical interface requires a 3.3V supply and the signaling is DC-coupled into the interface media. This severely restricts technology migration into low-voltage silicon applications that will be found in future PCs and other multi-media applications.
HDMI is well-suited for its application to TVs, but falls short for broad cross-industry application support due to limitations in terms of performance scalability. HDMI also suffers from being a closed and proprietary standard with stringent compliance testing and licensing issues. Although HDMI does provide true multi-media support and optional content protection, being a super-set of DVI it exhibits similar operational constraints. Like DVI, HDMI cable is bulky and exceedingly expensive, although the HDMI connector design is substantially smaller than the DVI connector. HDMI cable lengths are typically restricted and interoperability issues with the interface are very common.
DisplayPort provides an open, extensible, and scalable multi-media digital interface targeted as a replacement for existing interfaces in PC-to-display and inside-the-box applications. It is, however, focused mainly on PC-to-display connectivity, much as HDMI is focused on consumer electronics multi-media applications. DisplayPort also suffers from limited cable length with a typical three meters for full bandwidth support. Although limited interoperability tests have been carried out, wide-scale deployment of DisplayPort will be required to prove or disprove the technology claims of robustness. Connector and cabling costs and performance are therefore unknown at this time.
The SDI interface has enjoyed over 20 years of development, evolution, and continuous interoperability testing in professional applications. This open standard interface provides multi-media connectivity over a coaxial cable connection that does not suffer from the cable length limitations, routability, interoperability, or high connectivity costs of the other digital interfaces described herein. The almost exclusive use of SDI in professional applications does, however, mean that the need for content protection and bi-directional command and control information has not as of yet been addressed in the interface.
Among the aforementioned standards, only SDI operates on a single coaxial copper link. The DVI, HDMI, and DisplayPort interfaces all employ bundles of different cables, typically comprising several twisted pairs accompanied by additional single wires, all enclosed in an outer insulator layer. The electrical and physical characteristics of these interfaces have led to performance disadvantages in achieving cable lengths even far shorter than that of SDI.
The electrical characteristics and physical media for DVI, HDMI and DisplayPort utilize low voltage differential signaling (e.g., transition minimized differential signaling or TMDS) over multiple twisted pair copper cables. As the data rate requirements for these interfaces extend into the multiple Gb/s range, the limitations of the signaling and twisted pair copper cable media have an increasingly detrimental effect on cable reach and reliability of operation. These limitations are due to two major sources of signal degradation: (1) attenuation; and (2) intra-pair skew.
The loss characteristic 10 of a typical cable used in HDMI, DVI, or DisplayPort applications is shown in FIG. 1. In this figure, four different lengths of cable (2.5 m, 5 m, 10 m and 20 m) are described, and the attenuation loss of the cable, measured in dB, is plotted as a function of frequency. Also shown in this figure as dashed vertical lines, are the frequencies of most interest in current HDTV standard implementations, including 720p/1080i, 1080p and 1080p Deep Color. As shown in this figure, as the cable lengths extend beyond 5 m, losses become substantial.
In addition to losses caused by attenuation in the cable, signal loss in the HDMI, DVI, or DisplayPort interface technologies can also be attributed to the effects of intra-pair skew. FIG. 2, for example, illustrates cable attenuation as a function of intra-pair skew for a typical cable. It should be noted that this loss is not included in the diagram of FIG. 1 and at higher data rates, significant additional attenuation is added to the losses illustrated therein.
The application of transmitter pre-emphasis and/or receiver cable equalization is widely deployed to improve performance of these interfaces, and at data rates greater than about 1.5 Gb/s per signaling lane, application of these technologies is instrumental. This rate is significant as it represents the transmission of full high-definition video (1920×1080p60 RGB 444 10-bit). By utilizing pre-emphasis and/or receiver equalization, reliable operation at this rate can typically be achieved for cable lengths in the range of about 5 to 10 meters. Improvements in cable manufacturing technologies and the use of thick (24 AWG) twisted pair cables with multiple layers of shielding can further improve connectivity by reducing signal attenuation and controlling intra-pair skew. Using these additional techniques, cable lengths of up to 15 meters can be achieved for full HD transmission, but these cables are very expensive and generally very bulky and difficult to install and manage.
In multi-media interfaces, such as described herein, there are typically other lower rate auxiliary, configuration and/or control channels in addition to the higher rate digital content channels that may be transmitted between the source and the sink. Link and device management signals fall under this category. In the existing interfaces, often this information is of a bi-directional nature where the receiver and the transmitter time-share a dedicated link for this purpose. The DDC link in the DVI and HDMI interfaces and the auxiliary channel in the DisplayPort interface are examples that require bi-directional communication. These lower rate links are referred to herein, generally, as auxiliary channels. In addition to the bi-directional links, there may be a need for very slow and unidirectional communication as well. The hot plug detect signal is an example of such links in the DVI, HDMI, and DisplayPort interfaces. These types of links are referred to herein as status channels. In addition, a power link may be provided that offers one end (usually the receiver) with the required supply current provided by the other end (usually the transmitter). This calls for a DC supply connection (hereinafter, a supply channel) between the two sides of the link.